The invention relates to a process for the preparation of carboxylic anhydrides by carbonylation of carboxylic esters or of ethers. The invention relates in particular to the preparation of acetic anhydride from methyl acetate or dimethyl ether.
British Patent Specification No. 1,538,783 describes a process for the preparation of carboxylic anhydrides by carbonylation of a carboxylic ester or of an ether in the presence of a group VIII noble-metal compound, an iodide or bromide, a promoter and an organic nitrogen or phosphorus compound in which the nitrogen or phosphorus is trivalent. Group VIII noble metals are, however, expensive and there is a need for a process in which cheaper catalysts, such as, for instance, nickel catalysts, can be used with the conversion desired being nonetheless achieved under moderate reaction conditions at a rate suitable for use on a commercial scale. Carbonylation of esters or ethers in the presence of nickel complexes has been described in U.S. Pat. No. 2,729,651. However, the reaction is carried out under very high pressures.
As appears from U.S. Pat. No. 4,002,678, it is possible to employ lower pressures by carrying out the carbonylation in the presence of an iodide or bromide, an organic nitrogen or phosphorus compound in which the nitrogen or phosphorus is trivalent and a catalyst comprising both a nickel and a chromium component. In most examples, a reaction temperature of 150.degree.-155.degree. C. is used.
European Patent Specification No. 48,210 points out that the reaction rate of the process according to U.S. Pat. No. 4,002,678 is low and a process is described in which carbonylation is carried out in the presence of a nickel catalyst, an alkyl or acyl iodide, a sulphone as solvent and an alkali metal salt, alkaline earth metal salt, quaternary ammonium iodide or quaternary phosphonium iodide as co-catalyst. The reaction is, however, effected at a considerably elevated temperature of at least 180.degree. C. According to Example 9, use of an elevated temperature appears to be essential for a reasonable conversion rate to be achieved. In that example, a mere 5% conversion of the methyl acetate is achieved at a temperature of 160.degree. C.
European Patent Specification No. 55,622 likewise describes a process for the preparation of acetic anhydride by carbonylation of methyl acetate or dimethyl ether, the aim being to cause the reaction to proceed at as high a rate as possible. The carbonylation is effected in the presence of nickel or a nickel compound, an iodide or bromide, an organic nitrogen compound and a co-catalyst comprising one or more compounds of metals in groups IA, IIA, IIIA and IVA of the "Periodic System". (The "Periodic System" to which reference is made in this specification is reproduced in the "Handbook of Chemistry and Physics" 45th edition 1964-1965, p. B). With this process too, a temperature of at least 170.degree. C. and usually 180.degree.-200.degree. C. is used.
It has now been found that, in the preparation of carboxylic anhydrides by carbonylation of an ester or ether in the presence of a nickel catalyst, an iodide and/or bromide source and a co-promoter (or co-catalyst), a further rise of the reaction rate can be achieved by carrying out the reaction in the presence of a compound having the formula I stated hereinafter as promoter. Said rise also occurs in the presence of sulphones or of the co-promoters such as quaternary ammonium iodides and alkali metal compounds recommended in European Patent Specifications Nos. 48,210 and 55,622 for increasing the reaction rate. Even at relatively low temperatures, such as for example 165.degree. C., the compound having the formula I increases the reaction rate to a considerable degree. This is, inter alia, of interest because catalyst stability is generally greater at lower temperatures. The enhancement of reaction rate also occurs if only small quantities of nickel catalyst and/or iodide or bromide are used and if there is no (additional) solvent. When the process is effected on a commercial scale, it is naturally an important advantage if the quantity of catalyst to be recovered and the quantities of iodide and/or bromide, such as methyl iodide, and solvent can be kept low. If the examples stated hereinafter, reaction rate is consequently expressed in g carboxylic anhydride/g Ni/g I or Br/hour. This mode of expression is, moreover, more correct because reaction rate is generally directly proportional to both the quantity of Ni and the quantity of I or Br present in the reaction mixture. With the process according to the invention, very high reaction rates of, for example, 20 g carboxylic anhydride/g Ni/g I/hour can be achieved.